This invention relates generally to susceptors used in chemical vapor deposition of material on semiconductor wafers, and more particularly to a susceptor having a baffle for use in controlling material deposition thickness.
Chemical vapor deposition is a process by which a stable solid may be formed by decomposition of chemical vapors using heat, plasma, ultraviolet light or other energy sources. Chemical vapor deposition is widely employed in the production of semiconductor wafers to grow epitaxial layers on a surface of the wafers, as well as for the deposition of polysilicon on the wafers. Epitaxy is an important process in the semiconductor material industry for achieving the necessary electrical properties of the semiconductor material. For example, a lightly doped epitaxial layer grown over a heavily doped substrate permits a CMOS device to be optimized for latch up immunity as a result of the low resistance of the substrate. Other advantages, such as precise control of the dopant concentration profile and freedom from oxygen are also achieved.
Epitaxial growth is almost universally carried out by chemical vapor deposition because it is the most flexible and cost efficient method for growing epitaxial layers on semiconductor material. Generally speaking, chemical vapor deposition involves introduction of volatile reactants (e.g., SiCl.sub.4, SiHCl.sub.3, SiH.sub.2 Cl.sub.2 or SiH.sub.4) with a carrier gas (usually hydrogen) in a reactor. Achieving the desired epitaxial growth on the semiconductor material is also temperature dependent. Depending upon which reactant is used, the temperature in the reactor may be from 950.degree. C. to 1250.degree. C. The environment in which the deposition occurs must be clean with the oxygen content below 1 ppma.
Chemical vapor deposition is carried out in reactors which either pass the reactants and carrier gas generally parallel to the face of the semiconductor wafers (horizontal or cylindrical reactors) or generally perpendicularly to the face of the wafers (vertical reactors). For various reasons understood by those of ordinary skill in the art, cylindrical or barrel reactors are commonly employed in the industry. A barrel reactor has silicon carbide coated, graphite susceptor which is generally in the form of a polygonal tube having walls in which there are generally vertically arranged, circular recesses for receiving semiconductor wafers. The walls of the susceptor are sloped slightly from the vertical to permit the wafers to be retained in the recesses. The reactants and carrier gas are introduced generally at the top of the reactor and flow generally downwardly and then up the opposite side. The vapor flow is made more complex by interaction of the vapor with the walls of the reactor and the rotation of the susceptor in the reactor, but the flow of reactants is always substantially parallel to the faces of the wafers.
One of the primary problems associated with chemical vapor deposition in barrel reactors is maintaining the thickness uniformity of the deposited layer of material on each wafer and between the various wafers held on the susceptor. The most common method of controlling thickness uniformity is the adjustment of vapor flow, although minor changes can be made through temperature adjustments. However, it has been found that these adjustments are not sufficient to obtain thickness uniformity when processing relatively large semiconductor wafers (e.g., 200 mm diameter wafers). More specifically, too much material tends to be deposited at the bottom of large diameter wafers held in the lowest recess on each susceptor wall.